Electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid vehicles (PHEV) have been suggested as a means to reducing fossil fuel consumption and the potential adverse environmental implications. PHEV technology is attractive because it offers potential benefits of an all-electric vehicle, while preserving the range and flexibility of an internal combustion engine powered vehicle.
HEV and PHEV systems are not able presently to provide all power demands, for example during heavy acceleration, because of the power supply limitations of the battery technology. Additionally, the battery technology itself still adds significantly to vehicle mass and consumes a substantial portion of storage space within the vehicle.
The design of an electrical energy storage system for an electrified drivetrain of an electric or hybrid electric vehicle poses significant challenges. Existing vehicle electrical energy storage systems, primarily single chemistry lead-acid, nickel metal hydride, lithium and the like are inadequate. Other electrical energy storage technology, such as employed in small scale applications like consumer electronics, can inform the design of energy storage systems for electrified drivertrain leaves many questions unanswered. The suggestion of a battery structure, i.e., chemistry, cell configuration, construction, size and shape, suitable for consumer electronics, for example, may not scale to provide a solution to the vehicle energy storage system designer.
Design issues including cell and module robustness, safety, aging, lifetime, thermal effects, material/shelf life, shock and vibration resistance and general suitability in a vehicle environment all come into play. Issues of system scale also exist. Load requirements in a consumer electronic device may be relatively small, while electrified drivetrain systems may demand power delivery in the 20-100 kiloWatt range at 300-400 volts and significantly high load currents of 200 amps or more. A vehicle environment is also an extreme use environment subjecting the system to temperature extremes, changing temperatures, shock and vibration and, of course, crashes. Costs including initial installation and future replacement are also of concern. Selection and usage of materials should be made with a view toward sustainability, i.e., use of materials that are fundamentally abundant and reusable.
The typical energy storage/battery system of electric (EV), hybrid electric (HEV) or plug-in hybrid (PHEV) electric vehicle is limited to a single storage system or module with a single cell chemistry and architecture. To meet the many diverse operating conditions the vehicle might experience, the designer necessarily compromises in selecting the battery system. The resulting drawbacks are less than optimal energy delivery, volumetric size, weight and operating complexity (number and configuration of cells, cell monitoring for health and failure, etc.).